How to Replace Thermostats on a 2006 Mercury 150 EFI Two-Stroke

Quick Answer: Replace the thermostats on a 2006 Mercury 150 EFI two-stroke only after confirming the engine family and serial number. Let the engine cool, disconnect the battery, remove the thermostat covers, retain the factory orientation of each thermostat and seal, clean the housing faces without gouging them, install serial-matched parts, and torque the covers to the exact service-manual value. Then test on cooling water and stop immediately if an overheat warning returns.

Applies to: 2006-era Mercury 150 EFI two-stroke V6 outboards with paired cylinder-bank thermostats. This does not apply to the later 150 FourStroke, DFI, or Verado families. Thermostat design, opening specification, seal arrangement, cover hardware, and torque can change by serial range.

What Does the Thermostat Do on a Mercury 150 EFI?

Each thermostat restricts coolant flow while a cold engine warms, then opens as temperature rises. Correct warm-up supports stable combustion and even cylinder-bank temperature. A thermostat stuck closed can contribute to overheating; one stuck open can delay warm-up and create uneven operation. Both conditions deserve diagnosis rather than automatic parts replacement.

The visible condition of the removed parts can be useful evidence. Heavy mineral deposits, corrosion around the moving element, a distorted seal, or debris in the housing indicate that cooling-water quality and flushing practices should be reviewed along with the thermostat itself.

Which Symptoms Point to a Thermostat Problem?

• An overheat warning after the engine has been running normally.
• A cylinder bank that warms differently from the other bank.
• Persistent cold running, rough low-speed behavior, or excessive warm-up time.
• Salt or mineral residue around a thermostat cover.
• A thermostat that is visibly stuck, corroded, or contaminated when removed.

These symptoms are not proof by themselves. A damaged impeller, blocked intake, obstructed water passage, pinched hose, failed temperature sensor, or other cooling-system fault can produce similar behavior. If the engine has overheated severely or repeatedly, a complete cooling-system diagnosis is safer than replacing one component and restarting.

For an overview of the pump side of the system, see JLM's Mercury water-pump maintenance guide.

How Do I Choose the Correct Mercury 150 Thermostat?

Start with the engine serial number, not only “150 HP” or “2006.” Mercury sold several 150-horsepower families with different cooling components. Confirm whether the engine is EFI two-stroke, DFI, FourStroke, or another configuration, then match the thermostat, seal, and any cover gasket to the appropriate parts diagram.

Browse the Mercury 150 HP parts collection and narrow the search in the Mercury thermostat collection. Do not rely on a product photograph alone: compare replacement numbers, engine family, displacement, horsepower range, and serial range.

Pro Tip: Order every seal required for both cylinder banks before opening the engine. Reusing a flattened or hardened seal can create a leak even when the thermostat itself is correct.

What Tools and Parts Do I Need?

• Two serial-matched thermostats when the engine uses one per cylinder bank.
• New thermostat seals, O-rings, or cover gaskets specified for the exact model.
• Six-point sockets and a calibrated inch-pound or low-range torque wrench as required.
• Plastic gasket scraper, nylon brush, clean towels, and a small parts tray.
• Approved cooling-water supply for the post-service test.
• The factory service procedure containing orientation, opening specification, and torque.

Work only on a cold engine. Remove the ignition key, disconnect the battery, and keep loose tools away from the flywheel and electrical terminals. Expect some residual water when a cover opens; protect electrical connectors and clean spills promptly.

How Do I Remove the Thermostat Covers?

1. Remove the cowling and locate the thermostat cover on each cylinder bank using the serial-specific diagram.
2. Photograph hose routing, cover position, and surrounding connectors before moving anything.
3. Move only the hoses or retainers required for straight access; do not pull on wiring.
4. Loosen each cover fastener gradually. Hold the cover so it does not spring away or drop hardware.
5. Lift the cover straight off and record how the thermostat and seal sit in the bore.

If a cover is stuck, tap it gently with a non-marring tool. Do not lever a screwdriver against the sealing face. Stop if a bolt binds heavily; a broken fastener in the cylinder head turns a short service into a difficult repair.

What Should I Inspect After Removing the Thermostat?

Compare the two removed thermostats side by side. Look for corrosion, mineral crust, foreign material, a damaged spring or element, and differences in how the moving valve sits. Inspect the rubber sealing area for flattening, cuts, swelling, or hardening.

Next, inspect the housing bore and cover. White or green deposits point to salt or mineral exposure; pitting on a sealing surface may prevent a new seal from working. Check water passages for loose debris without pushing contamination deeper into the engine.

Pro Tip: Keep the port and starboard parts in separate labeled trays. Their condition can reveal a bank-specific restriction, and mixing them removes that diagnostic clue.

Can I Test the Old Thermostats?

A controlled hot-water test can show whether a removed thermostat begins to move and returns as it cools. Use a thermometer, suspend the part so it does not touch the container, and compare behavior with the opening specification in the correct Mercury literature. Do not use an unverified internet temperature or apply direct flame.

Testing is diagnostic, not a reason to reinstall a visibly corroded, damaged, or unreliable part. A thermostat may move on the bench yet bind intermittently in service. If one unit differs materially from the other, investigate the associated bank for blockage or abnormal deposits.

How Do I Clean the Thermostat Housing Safely?

1. Place a clean towel below the opening to catch loosened residue.
2. Use a plastic scraper and nylon brush on the accessible sealing face.
3. Remove old gasket or seal material without rounding the bore or scratching aluminum.
4. Lift debris outward; do not sweep it into the cooling passage.
5. Wipe the surface dry and inspect it under good light.

A deeply pitted housing, damaged cover, or stripped thread needs repair before reassembly. Sealant is not a substitute for a sound surface, and extra sealant can squeeze into the cooling circuit. Use only the sealing method specified for the serial range.

How Do I Install the New Thermostats and Seals?

1. Compare each new thermostat and seal with the removed part and verified parts diagram.
2. Install the seal in the exact groove or position specified; do not twist or stretch it.
3. Place the thermostat in the recorded direction. Do not assume both ends are interchangeable.
4. Position the cover squarely without disturbing the thermostat or pinching the seal.
5. Start every fastener by hand, tighten evenly in stages, and finish at the serial-specific torque.

Do not copy torque from a different Mercury 150 generation. Small cover bolts thread into aluminum and are easy to strip. A torque wrench and the correct specification are part of the repair, not optional finishing touches.

How Do I Test the Repair?

1. Restore every hose, retainer, and electrical connector to its recorded position.
2. Reconnect the battery and provide cooling water using an approved method.
3. Start the engine and immediately verify the normal cooling-water indicator stream.
4. Watch both cover areas for leakage while the engine warms.
5. Confirm stable operation and no overheat warning; use approved diagnostic temperature measurement if required.

Stop immediately if there is no water flow, a cover leaks, one bank heats abnormally, or the warning returns. Recheck thermostat orientation, seal placement, cover seating, water-pump condition, hoses, passages, and sensors before further operation.

Common Mistakes to Avoid

• Ordering by horsepower alone and receiving a thermostat for another 150 family.
• Opening a hot, pressurized cooling system.
• Mixing the removed port and starboard parts before comparing them.
• Scratching the aluminum sealing face with a steel scraper.
• Reusing flattened seals or adding unapproved sealant.
• Installing the thermostat backward or pinching its seal.
• Guessing the cover-bolt torque.
• Assuming every overheat warning is caused by the thermostat.

Where Can I Find Mercury 150 Thermostat Parts?

The JLM collection contains parts for multiple Mercury 150 configurations. The collection link is a starting point, not proof of fitment. Match the engine identification and replacement number before purchase.

Frequently Asked Questions

Does a 2006 Mercury 150 EFI two-stroke use two thermostats?

The V6 arrangement discussed here uses paired cylinder-bank thermostats, but confirm the exact engine and serial range before ordering. Other Mercury 150 families use different layouts.

Should I replace both thermostats at the same time?

When both are the same age and service history, replacing them as a matched pair avoids leaving one bank controlled by an older part. Keep the removed units separated long enough to compare their condition.

Can a bad thermostat cause a Mercury 150 to overheat?

Yes, a thermostat stuck closed can restrict flow, but pump damage, blockage, hoses, sensors, and other faults can create the same warning. Diagnose the complete cooling path.

What temperature should the thermostat open?

Use the value in the serial-specific Mercury service literature or verified part specification. Do not apply a temperature taken from another 150-horsepower engine family.

Why does the new thermostat cover leak?

Likely causes include an incorrect or pinched seal, a dirty or pitted mating surface, a warped cover, wrong hardware, or uneven tightening. Stop the engine, cool it, and correct the sealing fault.